Quick Question about turbo talk?

I know this will sound really stupid but when people talk about turbos they always say like To3/To4e or watever...I know that the numbers are the different sizes between the compression wheel and the exhaust wheel. So my questions/questions are...What does the "T" stand for(turbo?)? What do the letters stand for? And how exactly are the wheels,blades, whatever, exactly measured? Thanks for ANY info!